Should I spell out numbers (e.g. twenty) or simply give the number (e.g. 20)?

Should I spell out numbers (e.g. twenty) or simply give the number (e.g. 20)?

According to the Seventeenth Edition of the Chicago Manual of Style, the general rule is to spell out whole numbers from zero through one hundred. For any number beyond one hundred (e.g. 220), you can simply provide the number.